1. Field of the Invention
The present invention relates to a mixing method and mixer for mixing polymer dope, and a solution casting process and apparatus. More particularly, the present invention relates to a mixing method and mixer for mixing polymer dope with high efficiency even in a simple structure, and a solution casting process and apparatus.
2. Description Related to the Prior Art
Cellulose acylates are used as a support of polymer film contained in photosensitive materials, such as photographic films, owing to advantageous characteristics, for example rigidity, non-flammability, and the like. A typical example of cellulose acylate is cellulose triacetate (TAC) having an average acetylation degree of 57.5-62.5%. Also, the polymer film of the cellulose triacetate (TAC) is used as a protection film of a polarizing element, or an optical compensation film (view angle enlarging film or the like), any of those being incorporated in a liquid crystal display (LCD) panel. This is effective because of optically utilizing the highly isotropic property of the polymer film.
Typical examples of producing method for the polymer film include extrusion and solution casting. In the extrusion, polymer is heated and melted, and extruded by an extruder to produce the polymer film. The extrusion is characterized in high productivity and a low manufacturing cost. However, a thickness of the polymer film is difficult to adjust in the extrusion, which is not suitable for producing optical film due to occurrence of die lines on the polymer film. In the solution casting, polymer dope is used and constituted by polymer and a solvent. The polymer dope is cast on a casting support, and stripped from the support when a self-supporting property develops. The stripped self-supporting cast film is dried, and wound as the polymer film. The solution casting is capable of producing the polymer film with high isotropic property, with regularity in the thickness, and without foreign material in comparison with the extrusion. The solution casting is utilized specifically for producing the polymer film of optical use.
In general, additives are used additionally in the polymer dope for the solution casting and mixed with the polymer and the solvent. The use of the additives in the polymer dope is effective, because retardation control agents can adjust the optical performance. Flame retardants impart such important property to the polymer film as non-flammable property as important property. Release agents can be used for raising productivity in the course of manufacture.
To prepare the polymer dope for casting, liquid additive having the additives is mixed with the polymer dope as solution of the polymer in the solvent. Examples of mixing methods include in-line mixing and batch mixing or tank mixing. In the in-line mixing, the polymer dope is supplied through a flow line continuously to mix the additives with the polymer dope. In the batch mixing or tank mixing, the additives are mixed with the polymer dope stored in a tank. The in-line mixing is generally used in the solution casting for mixing the additives with the polymer dope.
Examples of mixers for the in-line mixing of liquid additive with the polymer dope include a dynamic mixer and a static mixer. The dynamic mixer includes a stirring blade for rotating. The static mixer mixes fluid without using such a movable element. In the dynamic mixer, a shaft for the stirring blade must be kept rotatable smoothly with suitable lubricant property. Also, the dynamic mixer requires a sealing property to prevent leakage of the polymer dope or the additives through a gap near to the shaft. It is known to use oil or lubricant as sealant for requirements of both of the lubricant property and sealing property. However, an ideal structure of satisfying the requirements at the same is extremely difficult to create. When the dynamic mixer is used, unwanted mixture of the lubricant is likely to occur with the polymer dope as foreign material at the time of stir. The contamination will result in lowering the suitability of the polymer film for shaping, and lowering the optical performance. In contrast, the static mixer can operate without such problem of contamination in the dynamic mixer, because of lack of a movable element.
Accordingly, the static mixer is used for mixing the additives with the polymer dope. U.S. Pat. Pub. No. 2008/056064 (corresponding to JP-A 2006-076280) discloses an example of the static mixer. A twisting type of static mixer includes twisted plates for mixing fluid by a flow in a curved passage in a flow line. A Sulzer mixer includes plural crossed plates combined to cross one another alternately, to split the fluid in plural flows in the flow line. The flowing fluid is split and moved in reverse in a repeated manner, to mix the additives with the polymer dope. It is possible to optimize the mixing of the additives with the dope by modifying or adjusting the positions, number and the like of the plates in the static mixer.
In the solution casting to produce the polymer film without unevenness in the optical performance, uniformity of the polymer dope is required. The additives is mixed with the polymer dope by connecting approximately 5-10 static mixers serially. However, a problem arises in the large space of installation and complexity in the maintenance due to the serial arrangement of the static mixer in a great number.
Recent development of the liquid crystal display panel is remarkable so that there are many new types of liquid crystal display panels, such as TN type, VA type and the like. The polymer film must be produced to have specifics suitable for the various types of liquid crystal display panels. Improvement related to the efficiency in producing the polymer film is an important concern to manufacturers of the polymer film. However, a great number of the static mixer must be optimized at each time of a change in the polymer dope or composition of the additives typically in a system for manufacturing various types of polymer films. Efficiency in the manufacture of the polymer film will be lower due to complexity in operation for the optimization of mixing.